eso0123 — Photo Release

Aurorae and Volcanic Eruptions

Thermal-IR Observations of Jupiter and Io with ISAAC at the VLT

7 June 2001

Impressive thermal-infrared images have been obtained of the giant planet Jupiter during tests of a new detector in the ISAAC instrument on the ESO Very Large Telescope (VLT) at the Paranal Observatory (Chile). They show in particular the full extent of the northern auroral ring and part of the southern aurora. A volcanic eruption was also imaged on Io, the very active inner Jovian moon. Although these observations are of an experimental nature, they demonstrate a great potential for regular monitoring of the Jovian magnetosphere by ground-based telescopes together with space-based facilities. They also provide the added benefit of direct comparison with the terrestrial magnetosphere.

Aladdin Meets Jupiter

Thermal-infrared images of Jupiter and its volcanic moon Io have been obtained during a series of system tests with the new Aladdin detector in the Infrared Spectrometer And Array Camera (ISAAC), in combination with an upgrade of the ESO-developed detector control electronics IRACE. This state-of-the-art instrument is attached to the 8.2-metre VLT ANTU telescope at the ESO Paranal Observatory.

The observations were made on November 14, 2000, through various filters that isolate selected wavebands in the thermal-infrared spectral region [1]. They include a broad-band L-filter (wavelength interval 3.5 - 4.0 µm) as well as several narrow-band filters (3.21, 3.28 and 4.07 µm). The filters allow to record the light from different components of the Jovian atmosphere (mostly greenhouse gases and aerosols) and the appearance of the giant planet is therefore quite different from filter to filter.

At the time of these observations, Jupiter was 610 million km from the Earth and 755 million km from the Sun. The angular size of its disk was 48 arcsec, or about 40 times smaller than that of the full moon.

The ISAAC instrument

The ISAAC multi-mode instrument is capable obtaining images and spectra in the near-to-mid infrared wavelength region from 1 - 5 µm. It is equipped with two state-of-the-art detectors, a Hawaii array (1024 x 1024 pix 2 ; used in the 1.0 - 2.5 µm spectral region) and an Aladdin InSb array also with 1024 x 1024 pix 2, and sensitive over the entire 1 - 5 µm region, but for the time being only used for the 3-5 µm region.

Observations in the thermal-IR wavelength region with the Aladdin array rely on the 'chopping' technique. It consists of tilting the telescope's lightweight 1.1-m secondary mirror back and forth ('tip-tilt') about once per second. This basic technique allows to subtract the strong infrared emission from the sky by also observing an area adjacent to the object area - the difference is then the radiation from the object.

Without this method, the strong and rapidly variable sky emission - that originates in all layers of the terrestrial atmosphere - and also the thermal emission from the telescope would render infrared observations of faint celestial objects impossible. 'Chopping' is further combined with 'nodding', i.e. moving the telescope in the direction opposite to the direction of the 'chop' in order to achieve better cancellation of residual sky emission.

Thanks to the very good stability provided by the VLT tip-tilt system and excellent seeing conditions, the image resolution obtained on these images is about 0.39 arcsec in the L-band. The field-of-view is 72 x 72 arcsec 2 (1 pixel = 0.07 arcsec) - this corresponds to 1.5 times the size of Jupiter's disk in November 2000. No other infrared astronomical instrument working at these wavelengths is capable of producing so sharp images over such a large field-of-view.

Some of these images are shown below. They were prepared and analysed by Jean Gabriel Cuby (ESO-Chile), Franck Marchis (CFAO/University of California, Berkeley, USA) and Renée Prangé (Institut d'Astrophysique Spatiale, Orsay, France).

Thermal-IR Views of Jupiter

The above images were obtained in different wavebands. The appearance of the planet depends on whether the filter corresponds to a spectral band in which auroral emission lines dominate over the polar haze continuous emission (for details, read the Addendum), e.g. in the narrow-band (NB) filters at wavelength 3.28 µm (ESO Press Photo eso0123c) and 3.21 µm (ESO Press Photo eso0123d).

In the filter bands where this is not the case, the contrast between the auroral ring and its surroundings is less prominent, as in the broad-band L-filter that covers the wavelength interval 3.5 - 4.0 µm; (ESO Press Photo eso0123a) and in the narrow-band filter at 4.07 µm (ESO Press Photo eso0123b).

There is also a dramatic difference in the brightness of Jupiter's atmospheric clouds. This effect is linked to the degree of absorption of the sunlight by a methane layer that varies very much with wavelength. For instance, the spectral band at 3.28 µm (ESO Press Photo eso0123c) is at the edge of a strong methane absorption band and the disk therefore appears very dark at this particular wavelength.

As explained above, the chopping technique must be applied to perform these observations. It is achieved by moving the 1.1-m secondary mirror of the ANTU telescope in the direction perpendicular to Jupiter's axis of rotation. The dark circles that cover the right part of the images of the planet are due to the fact that the chop throw is limited to 30 arcsec only. While this is quite sufficient for observations of other, smaller objects, it is less than Jupiter's angular diameter at the time of these observations (48 arcsec). For that reason, the image of the planet is subtracted from itself at the right edge.

The bright spot to the left of the planet is Io, the innermost of the large moons. Its shadow on Jupiter is well visible on ESO Press Photo eso0123b (4.07 µm) ESO Press Photo eso0123d (3.21 µm). The dark spot to the right on the images is a 'negative' image of Io, caused by the chopping and image subtraction.

Note that Io is moving towards the right during the observations. At the time of the observations, the rotation axis of Jupiter was tilted about 3° towards the Earth so that the North Pole is well visible. Moreover, the magnetic axis is inclined 9.6° to the rotation axis. Thus the northern auroral ring is fully on the Earth-facing hemisphere, while the coresponding southern ring is barely visible at the lower limb of the planet.

The auroral ring

ESO Press Photo eso0123e is a false-colour combination of the images presented in ESO Press Photos eso0123a-d, now showing the full disk after careful correction for the 'shadowing effects' of the chopping process, as explained above.

The auroral oval is well visible all the way around the pole. The visibility on the far side is enhanced because of the grazing angle of view: near the limb, the apparent brightness increases since the line of sight passes along a longer section of the emitting layer, whereby the number of emitting atoms in these directions increases. On the contrary, it more difficult to detect the faint ring at lower latitudes on the day-side disc, where the path length is shorter.

In fact, the front part of the auroral oval has never before been observed from the ground - so far it was only seen with the Hubble Space Telescope (HST). The present photo therefore highlights ISAAC's excellent image quality and high stability. Note also that it has been possible to resolve two separate arcs on the right side of the ring; this is normally only possible by means of observations from space.

Another interesting property of this image is the extension of the polar haze, here seen in blue colour. A comparison with the rotation (yellow arrow) and magnetic (white arrow) axes shows that the polar haze is centered on the rotation axis whereas its source, the auroral ring, is centered on the magnetic axis.

This observation therefore suggests the following interpretation: the atoms and molecules that make up the polar haze are continuously created at the footprint of the auroral magnetic field lines, i.e., below the auroral rings. They spread over both polar regions, much more so in longitude than in latitude. This bears witness to the important role of the zonal winds in the Jovian atmosphere (blowing along the same latitude) in transporting the haze material, much stronger than that of the meridional winds (along the same longitude), even at the high latitudes of the auroral region. Jupiter's rapid rotation (about 10 hours per revolution) obviously plays an important role in this.

A volcanic eruption on Io

Io, the innermost major satellite of Jupiter is one of the most remarkable bodies in the solar system. Volcanic activity on its surface was first discovered by the NASA Voyager 1 and 2 spacecraft during fly-by's in 1979. This is attributed to internal heating caused by tidal effects between Jupiter, Io and the other Galilean satellites. Apart from the Earth, Io is the only other body in the solar system that is currently volcanically active. The volcanism on this moon is the main source of electrically charged particles (plasma) in the magnetosphere of Jupiter.

A bright polar feature is visible on several ISAAC images of Io, obtained through a narrow-band filter at 4.07 µm, c.f. ESO Press Photo eso0123f. In this waveband, the effect of reflected sunlight is negligible and the image resolution is the best. Applying a basic filtering algorithm, the sharpness of this image was further enhanced. The recorded emission is found to correspond to the Tvashtar hot spot that was discovered by NASA Infrared Telescope Facility (IRTF) in November 1999 and observed simultaneously by the Galileo spacecraft during its I25 flyby.

Such outbursts normally have a short lifetime, less than 1 month, and a very high temperature, more than 1000 K (700 °C). However, the Tvashtar outburst is quite anomalous and has lasted more than one year. The temperature has been estimated at about 1000-1300 K (700-1000 °C); this range is typical for silicate-based volcanism observed on the Earth.

The Galileo spacecraft observed the onset of this eruption, and twice again this year. Monitoring of this event by means of ground-based telescopes, as here with ISAAC at the VLT or by the ADONIS Adaptive Optics system on the ESO 3.6-m telescope at La Silla, gives the astronomers a most welcome opportunity to follow more closely the temperature evolution of the eruption and hence provides excellent support to the space observations.

The forthcoming arrival on Paranal of NAOS (the adaptive optics system for the VLT) and CONICA (the connected IR camera equipped with an Aladdin detector) will lead to a significant improvement of the achievable image quality. It will be employed for a large variety of astronomical programmes and will, among others, allow the detection and frequent monitoring of a large number of hot spots on the surface of Io.

Notes

[1]: ISAAC registers (infrared) electromagnetic radiation at wavelengths between approx. 1.0 and 5.0 µm which we sense as heat. The human eye registers electromagnetic radiation (light) at shorter wavelengths, from about 0.4 to 0.7 µm.

More information

Addendum: About the Jovian aurorae and polar haze

Aurorae Borealis and Aurorae Australis ('Northern and Southern Lights') are observed on Earth as well as on Jupiter. They appear as wavy curtains of light that follow the magnetic field lines at high latitude and they surround the north and south magnetic poles as a permanent, but variable ring of light . The light is produced by the impact of energetic charged particles (electrons or ions lost by the magnetosphere) onto the top of the atmosphere where they excite the atmospheric atoms and molecules, mainly atomic (H) and molecular hydrogen (H 2).

While the emissions that are excited directly by particle collisions are radiated in the visible and the ultraviolet regions of the spectrum, it was discovered at the beginning of the 1990's that the auroral rings may also be detected in the infrared (IR) region of the spectrum. The reasons for this are also known. The auroral particles excite or ionize the atoms (and ions) in the atmosphere, creating in some areas large numbers of H 3 + ions. They also generate a huge amount of thermal energy - in fact, the total energy deposited in the Jovian aurorae is about 10 14 watts, or 1000 times more than in a typical terrestrial aurora. H 3 + ions are capable of radiating their energy in narrow spectral lines in the infrared part of the spectrum near 2 - 4 µm wavelength and, as shown by the present ISAAC images, this radiation can be detected with ground-based telescopes.

The importance of monitoring Jovian auroral emissions is that it allows to measure the activity of the Jovian magnetosphere and - with the help of magnetic field models - to map in detail the auroral structures and the motion of energetic particles in the magnetosphere. The capability to perform such studies from the ground with a quality approaching that from space now promises dramatic improvement in our understanding of Jovian auroral processes and, equally important, the possible to compare them with those on the Earth.

There are other light emission mechanisms on Jupiter than the aurorae. Clouds are present in Jupiter's stratosphere that efficiently reflect the sunlight and which are responsible for the overall brightness of Jupiter's disk. In addition, the polar regions are covered by a 'haze' which is particularly bright in infrared light. The very nature and the origin of this haze is still quite puzzling, although it is now generally agreed that it is a by-product of the auroral activity.

It has been suggested that the polar haze may at least partly consist of heavy hydrocarbon molecules, polymers and/or aerosols that are produced where incoming energetic particles from the magnetosphere enter the upper atmospheric layers. Alternatively, the amount of energy deposited in the auroral atmosphere is so large that violent upward winds are produced that carry atoms and molecules from the deep atmosphere into the stratosphere. While discrete narrow emission lines dominate the auroral infrared spectrum, the polar haze emits at all wavelengths (a spectral 'continuum').

Contrary to their Jovian counterparts, terrestrial Aurorae are directly related to solar activity. Since it is now near the maximum in the 11-year cycle, terrestrial aurorae are unusually frequent and intense, and may also be visible at lower geographical l.